"We absolutely should have commenced open disclosure earlier on. We failed in that regard," he said.

"We didn't want to give them (patients) more stress while they were in their treatment without fully understanding what happened ... we made a mistake."

NSW's chief health officer Kerry Chant also pointed the finger of blame at St Vincent's when she was grilled by the inquiry earlier on Tuesday.

"I believe that St Vincent's did not understand the nature and seriousness of the issue and therefore that factored into the way the incident was portrayed and the information provided," Dr Chant said.

"There was a fundamental failure to recognise that, which then flowed through the communication from that point. They did not act with sufficient urgency and did not put in place sufficient supports."

Earlier this month, St Vincent's chief executive Professor Schembri told the inquiry he had informed Dr Chant about the under-dosing and an external review into the matter last November - three months earlier than when Health Minister Jillian Skinner says she was reportedly told about the incident.

When asked why she did not alert NSW Health at once, Dr Chant claimed she was not made aware of the significance of the incident.

"I would have been assured in November that there was no ongoing health risk (to the patients)," Dr Chant said.

"My impression was that the main purpose of the call was to give me a heads-up about a possible media story."

Dr Grygiel defended his actions earlier this month, saying he decided on a level of chemotherapy dosage 12 years before medical guidelines were introduced in 2006.

He said there was no evidence to suggest the guideline doses would have led to a better outcome for patients.

Dr Grygiel continued to treat patients until November last year, after which he stepped down.